fix typo

python/paddle/v2/fluid/layers/math_op_patch.py

@@ -154,7 +154,7 @@ def monkey_patch_variable():
         ("__rpow__", "elementwise_pow", True),
             # for logical compare
         ("__eq__", "equal", False),
-        ("__lt__", "less_then", False),
+        ("__lt__", "less_than", False),
         ("__le__", "less_equal", False), ):
         setattr(Variable, method_name,
                 _elemwise_method_creator_(method_name, op_type, reverse))